Regenerative medicine, in which a body part or organ which has been rendered dysfunctional due to various diseases or traumatic injuries is replaced with a regenerated body part or organ, is showing promise as a next-generation medical technique to complement medical transplantation (Non-Patent Literature 1). In past research of regenerative medicine, advances have been made in stem cell transplantation therapy in which stem cells or precursor cells are transplanted into an injured tissue or a partially dysfunctional organ to restore its function.
In regeneration of two-dimensional tissue consisting of a single type of cells such as skin or corneal epithelial cells, cardiac muscle cells, a tissue regeneration technology in which cells are organized by culturing the cells in a sheet form using cell sheet technology is nearing practical application. Therein, it is now possible to regenerate a functional skin tissue by stratifying fibroblasts, which are mesenchymal cells, and skin epidermal cells to artificially reproduce a histologically-appropriate layer structure, and this technique has been clinically applied in the treatment of severe burns.
Meanwhile, it is known that in an organ, multiple types of functional cells take on a three-dimensional arrangement to express a unique function. Almost all organs are generated by interactions between epithelial cells and mesenchymal cells during the fetal period, and exhibit unique morphology and organ functions. In current regenerative medicine techniques, it is difficult to arrange multiple types of cells in a three-dimensional fashion, and a regenerative organ construct that can immediately function ex vivo has yet to be developed.
Recently, research is being conducted with the goal of organ regeneration by regenerating an organ germ and reproducing its developmental process for ectodermal appendages such as teeth and salivary glands and skin appendages such as hair follicles. These organs are not directly related to the maintenance of life, but they are known to fall into organ loss or dysfunction. As an example, mention may be made of tooth loss due to dental caries, injury, and tooth germ hypoplasia, salivary secretion disorder associated with aging, and hair loss due to male pattern baldness and hair follicular dysplasia. These kinds of organ loss or dysfunction have a large impact on QOL (quality of life), and thus high expectations have been placed on functional restoration by organ regeneration.
Generally, in mammals and birds, ectodermal skin appendages such as hair, feathers, and nails are ubiquitous in the skin and have species-specific functions such as survival and reproduction. In mammals, hair functions to retain body heat and protect against injury and ultraviolet rays. Further, in higher mammals such as primates, hair produces characteristic colors and patterns on the body surface, and this is believed to be useful in the appeal of rank and fertility within a reproductive population. Hair also produces differences in hair quality such as thickness, hardness, and color in accordance with its area or function on the body surface, and exhibits aesthetic and functional value when it exists in large numbers in a specific area. Particularly in humans, the color and quality of head hair holds social significance, and it is believed that changes thereto due to aging or illness have a large impact on an individual's QOL.
In order to establish hair follicle regeneration medical techniques sufficient for clinical application, the growth and elongation of hair in which the regenerative hair follicle has a normal tissue structure and the hair shaft is suitable for the transplantation site is necessary. Such ectodermal appendages including skin appendages such as hair are normally generated by interactions between epithelial and mesenchymal cells during the fetal period. A hair follicle, which is one kind of ectodermal appendages, repeats growth and regression (the hair cycle) over an individual's lifetime. The regeneration of a hair bulb during the growth period is known to be induced by a molecular mechanism similar to that in the nascent stage of the hair follicle organ. Also, the regeneration of a hair bulb during such hair cycle is believed to be induced by hair papilla cells, which are mesenchymal cells. In other words, in the growth period, hair follicle epithelial stem cells are differentiation induced by hair papilla cells, which are mesenchymal cells, to regenerate a hair bulb. Further, since niches of neural crest-derived stem cells exist in the bulge region and the region below the bulge region, it is believed that hair follicles keep multiple stem cell niches and function as a stem cell pool.
In the past, attempts at hair follicle regeneration have been made by regeneration of the hair follicle variable region by replacing the mesenchymal cells (hair papilla cells and dermal root sheath cells), neogenesis of the hair follicle by mesenchymal cells having hair follicle inducing ability, reconstruction of the hair follicle by epithelial/mesenchymal cells, and the like. Further, it was recently demonstrated by the present inventors that a regenerative hair follicle germ reconstructed from adult mouse whisker-derived bulge region epithelial cells and adult mouse whisker-derived cultured hair papilla cells by the organ germ method (for example, refer to Patent Literature 1) emulates normal development and can regenerate hair follicles and hair. However, when regenerating a hair follicle using a regenerative hair follicle germ derived from an adult mouse whisker, there has been a problem in that almost all of the regenerated hairs become white hairs.